A medicament which contains the complex compound cis-diamminedichloroplatinum(II) has recently been marketed as a chemotherapeutic agent against cancer. This compound, known by the International Nonproprietary Name (INN) of cisplatin has proved to be an extremly potent antitumor agent, especially in the treatment of testicular tumors, and also, for example, of ovarian tumors and of oat-cell carcinomas. The disadvantage of cis-platin is its relatively high toxicity. Its nephrotoxicity and its action leading to lasting damage to hearing are particularly serious. Renal damage and damage to hearing are found with considerable frequency after administration of only a single therapeutic dose. Besides the nephrotoxic and hematotoxic action, the long-lasting severe nausea and the retching associated therewith are, above all, also extremely unpleasant for the patient. Numerous other platinum complexes (German Offenlegungsschrift No. 2,445,418, German Offenlegungsschrift No. 2,837,237, German Offenlegungsschrift No. 2,626,559 and German Offenlegungsschrift No. 2,539,179) and complex compounds of other transition metals have recently been proposed as agents having a cytostatic action. In German Offenlegungsschrift No. 2,801,355, a brown amorphous complex which is obtained by reacting ascorbic acid with a titanium(III) compound and a copper(II) compound (in a molar ratio of 36:1:6) is said to have curative and prophylactic actions against, inter alia, leukemia. It has been reported that titanocene dichloride, zirconocene dichloride and hafnocene dichloride have an inhibiting action on ascitic Ehrlich tumors in mice [P. Kopf-Maier, B. Hesse and H. Kopf, J. Cancer Res. Clin. Onco., 96, 43 (1980)]. Dialkyl- tin-dihalide phenanthroline complexes and bipyridyl complexes have been said to have inhibiting actions on P388 leukemia in mice (A. J. Crowe and P. J. Smith, Chemistry and Industry, page 200, Mar. 1, 1980).